[FEATURE REQUEST] Optional switch for validating cached leases with the Vault server before returning them

[lkubb]

Is your feature request related to a problem? Please describe.
The LeaseStore is intended to handle operations on (cached) leases. The get function validates that the cached lease is not expired (and tries to renew it if necessary before returning).

If a lease is not due for renewal and has been revoked on the server, this is currently not caught.

Describe the solution you'd like
Introduce an optional switch check_server that causes the store to check with the server that the cached lease is still valid.

Describe alternatives you've considered
Do a manual lease lookup before using it.